In an increasing variety of white lighting applications it is desirable or even possibly required to control the spectral characteristic of the white light. There are many variations of light that appear white. Sunlight, for example, appears warmer than white light from a fluorescent fixture. Light from an incandescent bulb often appears somewhat reddish in color. Yet, humans perceive such lights as ‘white.’ Even for light that appears ‘white’ to the human eye, many applications call for different characteristics of the white light. Typical white light sources provide light of a fixed nature, so that it is often necessary to use a different lighting device for each different application. However, with the advent of modern light sources such as light emitting diodes (LEDs) and attendant controls, it is often desirable to change the spectral characteristic of white light from a particular device to suit different needs or desires of a user at different times. For example, at times a user may prefer a cooler light and at other times the user may prefer a warmer light more analogous to sunlight.
It has long been known that combining the light of one color with the light of another color creates a third color. For example, the commonly used primary colors Red, Green, and Blue of different amounts can be combined to produce almost any color in the visible spectrum. Adjustment of the amount of each primary color enables adjustment of the spectral properties of the combined light stream. Recent developments for selectable color systems have utilized LEDs as the sources of the different light colors.
Light emitting diodes (LEDs) were originally developed to provide visible indicators and information displays. For such luminance applications, the LEDs emitted relatively low power. However, in recent years, improved LEDs have become available that produce relatively high intensities of output light. These higher power LEDs, for example, have been used in arrays for traffic lights. Today, LEDs are available in almost any color in the color spectrum. More recently, LEDs have been increasing in popularity for more general lighting in residential and commercial lighting applications.
Traditional LEDs emitted primary light colors. Systems are known which combine controlled amounts of projected light from at least two LEDs of different primary colors. Control of the primary colors included in the combined output light allows the system to generate a wide range of colors in the output of the system, including many variations that appear at least substantially white to human observers.
The introduction of white light LEDs has allowed semiconductor lighting systems to enter the market for more traditional lighting applications without the need for combining light of so many different colors. However, the white light LEDs tend to be relatively cool or bluish to the human observer. To adjust the color, many systems combine the bluish white light LEDs with a LED of a warmer primary color, such as amber or red.
Some of these systems for white lighting tend to provide a relatively static color. For example, a feedback may be provided to enable the microcontroller to adjust the LED outputs to maintain a pre-set temperature of the overall system output. Other systems, however, have allowed the user to set the color of the system output.
For example, United States Patent Application 2006/0268544 A1 by Rains Jr. et al. teaches optical integrating chamber lighting using multiple color sources to adjust white light. The Rains Jr. system provides white light having a selectable spectral characteristic (e.g. a selectable color temperature) using an optical integrating cavity to combine energy of different wavelengths from different sources with white light. The cavity has a diffusely reflective interior surface and an optical aperture for allowing emission of combined light. Control of the intensity of emission of the sources sets the amount of primary color light of each wavelength added to the substantially white input light output and thus determines a spectral characteristic of the white light output through the aperture.
The objective of most systems for general lighting applications is to provide a desired quality of white light of a desired color characteristic, e.g. color temperature of a relatively long usage life. This intent applies even in systems that allow the user to select or tune the output color—it is still desirable when the user sets the color temperature of the white light for the system to produce an acceptable quality of the desired color temperature white light and to maintain the output performance for a long expected usage lifetime.
For example, a problem arises from long-term use of LED type light sources. As the LEDs age, the output intensity for a given input level of the LED drive current decreases. As a result, it may be necessary to increase power to an LED to maintain a desired output level. This increases power consumption. Further, LEDs may not be uniformly bright. In this regard, for a given drive current, light output may vary from chip to chip. As performance of the LEDs of different colors declines differently with age (e.g. due to differences in usage), it may be difficult to maintain desired relative output levels and therefore difficult to maintain the desired spectral characteristics of the combined output. The output levels of LEDs also vary with actual temperature (thermal) that may be caused by difference in ambient conditions or different operational heating and/or cooling of different LEDs. Temperature induced changes in performance cause changes in the spectrum of light output.
Another problem with existing multi-color LED systems arises from control of the overall system output intensity. In existing systems, to adjust the combined output intensity, e.g. to reduce or increase overall brightness, the user must adjust the LED power levels. However, LED spectral characteristics change with changes in power level. If the light colors produced by the LEDs change, due to a power level adjustment, it becomes necessary to adjust the modulations or driver output power to compensate in order to achieve the same spectral characteristic.
To address these issues, many systems utilize optical and/or temperature sensing as feedback to the microcontroller, to adjust the LED operation parameters to maintain a set output intensity and a set output spectral characteristic. Optical sensing has often used sensors configured to sense the overall intensity and/or to sense the intensity of red (R), green (G) and blue (B) light bands encompassing the RGB outputs of the system LEDs. While broadband filters can be used to sense white photons, there is a concern of differentiation from other colored LEDs in the fixture. For example, if there is green light contribution in the light output, the broadband filter may not accurately differentiate the source of the green light, since the white LED spectrum is broadband, and thus includes green.
Hence, a need exists for a technique to efficiently provide white light of a selectable characteristic, with a focus on efficiently provided desired white light performance. A related need exists to control the white light to achieve several color temperatures along the black body curve. A need also exists to efficiently estimate the white photons in order to provide feedback control for respective colored LEDs. Further, a need exists for a system that maximizes the utilization of every LED. Still further, a need also exists for a technique to effectively maintain a desired energy output level and the desired spectral characteristic of the combined output as LED performance decreases with age, preferably without requiring excessive power levels.